Autotrophic organisms can make all of their own amino acids. Other cells utilize many preformed amino acids. Humans and other higher animals require a number of essential amino acids in the diet. These essential amino acids are obtained directly or indirectly by eating plants. These essential amino acids include lysine, tryptophan, threonine, methionine, phenylalanine, leucine, valine and isoleucine.
Constructing proteins with higher nutritional value has been a long-sought goal of scientists. Traditionally, agricultural scientists concentrated on breeding plants with high nutritional yield. Typically, these new varieties were richer in carbohydrates but usually poorer in essential proteins than the wild type varieties from which they were derived.
Seed storage proteins represent up to 90% of total seed protein in seeds of many plants. They are used as a source of nutrition for young seedlings in the period immediately following germination. The genes encoding them are strictly regulated, being expressed in a highly tissue specific and stage specific manner. These genes are almost exclusively expressed in developing seed. Different classes of seed storage proteins may be expressed at different stages in the development of the seed. They are typically stored in membrane bound organelles called protein bodies or protein storage vacuoles.
A related group of proteins, the vegetative storage proteins, have similar amino acid compositions and are also stored in specialized vacuoles. These proteins are generally found in leaves instead of seeds. These proteins are degraded upon flowering, and are thought to serve as a nutritive source for developing seeds.
Cereal grains and legume seeds which are key protein sources for the vegetarian diet are generally deficient in essential amino acids such as methionine, lysine, and threonine. Therefore, there is needed means for improving the nutritional quality of these proteins.